High-strength thick steel plate excellent in low temperature toughness at heat affected zone resulting from large heat input welding

ABSTRACT

The present invention provides a high-strength thick steel plate having a plate thickness of 50 to 80 mm and a tensile strength of 490 to 570 MPa which is able to realize an excellent HAZ toughness even when welding with a heat input of 20 to 100 kJ/mm is conducted and is characterized by containing, by wt %, 0.03-0.14% of C, 0.30% or less of Si, 0.8-2.0% of Mn, 0.02% or less of P, 0.005% or less of S, 0.8-4.0% of Ni, 0.003-0.040% of Nb, 0.001-0.040% of Al, 0.0010-0.0100% of N, and 0.005-0.030% of Ti, where Ni and Mn satisfy equation [1], and the balance of iron and unavoidable impurities:
 
Ni/Mn≧10×Ceq−3 (0.36&lt;Ceq&lt;0.42)  [1]
where,  Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a high strength thick steel plateexcellent in low temperature toughness at heat affected zone(hereinafter referred to as an “HAZ”) used for ships, offshorestructures, medium/high rise buildings, bridges, and so forth, moreparticularly relates to a steel plate having a thickness of 50 mm ormore and a tensile strength of 490 to 570 MPa and having an excellentwelded joint even in a case where welding with a welding heat input of20 to 100 kJ/mm is conducted.

2. Description of the Related

In recent years, the demands on the material properties of steelmaterials for welding used in large sized structures such as ships,offshore structures, medium/high rise buildings, and bridges have becomeincreasingly tough. Especially, among these structures, the use of steelplates having large thicknesses exceeding 50 mm and having tensilestrength of around 570 MPa has been increasing. Further, in order topromote higher efficiency of the welding, for the welding of suchhigh-strength thick steel plates, one-pass welding by a large heat inputwelding process such as electro-gas welding, electo-slag welding, etc.has been investigated. The demands on the HAZ toughness have becomeincreasingly tough in the same way as on the toughness of the basematerial per se.

Many proposals have been made hitherto paying attention to the HAZtoughness of steel plate to which a large heat input welding process isapplied. For example, Japanese Unexamined Patent Publication (Kokai) No.55-026164 discloses an invention of securing fine Ti nitrides in thesteel so as to reduce the austenite grain size in the HAZ and therebyimprove the toughness. Further, Japanese Unexamined Patent Publication(Kokai) No. 03-264614 proposes an invention making use of complexprecipitates of Ti nitrides and MnS as transformation nuclei of ferriteso as to improve the HAZ toughness. Further, Japanese Unexamined PatentPublication (Kokai) No. 04-143246 proposes an invention making use ofcomplex precipitates of Ti nitrides and BN as precipitation nuclei ofgrain boundary ferrite so as to improve the HAZ toughness.

However, Ti nitrides end up becoming almost completely dissolved in thevicinity of the border with a welded metal in HAZ where the highesttemperature reached exceeds 1400° C. (hereinafter also referred to as a“weld bond portion”). As a result, there is a problem that the effect ofimprovement of the toughness is lowered. For this reason, in steelutilizing the Ti nitrides as described above, it is difficult to meetthe recent tough demands for the HAZ toughness or the requiredcharacteristics of the HAZ toughness in ultra-large heat input welding.

Steels containing Ti oxides as a method of improving the toughness inthe vicinity of this weld bond portion are being used in various fieldssuch as thick plates and steel shapes. For example, in the field ofthick steel plates, as described in the inventions disclosed in JapaneseUnexamined Patent Publication (Kokai) No. 61-079745 and JapaneseUnexamined Patent Publication (Kokai) No. 61-117245, steel containing Tioxides is very effective for improving toughness at the large heat inputweld portion, and is promising in application to high tensile steels.This principle is that the Ti nitrides, MnS, etc. precipitate in themiddle of temperature drop after welding using the Ti oxides stable evenat the melting point of the steel as precipitation sites, fine ferriteis generated using these as sites, and as a result the production ofcoarse ferrite harmful to the toughness is suppressed and deteriorationof the toughness can be prevented.

However, such Ti oxides involve a problem that the number of particlesdispersed into the steel cannot be increased that much. The reason isthe coarsening or aggregation of the Ti oxides. It is believed that ifthe Ti oxides particles are increased, coarse Ti oxide particles of 5 μmor more, i.e., so-called inclusions, end up increasing. These inclusionsof 5 μm or more size become initiation sites for fracture of a structureor cause a drop of the toughness and therefore are harmful. Therefore,this should be avoided. For this reason, in order to achieve a furtherimprovement of the HAZ toughness, it was necessary to make use of oxideswhich are hard to coarsen and agglomerate and are more finely dispersedthan Ti oxides.

Further, many such methods of dispersion of such Ti oxides into steelare based on adding Ti into molten steel substantially not containingany Al or other strong deoxidizing elements. However, it is difficult tocontrol the number and degree of dispersion of Ti oxide particles insteel by just adding Ti into the melt. Further, it is also difficult tocontrol the number and degree of dispersion of precipitates such as TiNand MnS. For this reason, in steel in which Ti oxide particles aredispersed by only Ti deoxidation, there was the problem for example thatneither a sufficient number of Ti oxide particles nor a stable toughnessof thick plate in the thickness direction could be obtained.

With respect to such problems, Japanese Unexamined Patent Publication(Kokai) No. 06-293937 and Japanese Unexamined Patent Publication (Kokai)No. 10-183295 disclose inventions making use of Ti—Al complex oxides andTi, Al, and Ca complex oxides produced by the addition of Al immediatelyafter the addition of Ti or the complex addition of Al and Ca. By suchinventions, it became possible to greatly improve HAZ toughness in thelarge heat input welding.

SUMMARY OF THE INVENTION

However, with the conventional means of reducing the austenite grainsize of the HAZ or generating ferrite by using precipitates astransformation nuclei of the ferrite, it is necessary to increase thealloy elements in order to secure a tensile strength of 490 MPa or morewhen the plate thickness is 50 mm or more. In this case, the hardness ofthe HAZ rises and, at the same time, the production of MA(martensite-austenite constituent) degrading the toughness becomesremarkable. Therefore, a sufficient HAZ toughness such as the E grade(−20° C. guarantee) in for example the shipbuilding field cannot bestably secured. In addition, when the tensile strength becomes more than570 MPa, the required HAZ toughness cannot be obtained.

Therefore, an object of the present invention is to provide ahigh-strength thick steel plate excellent in the low temperaturetoughness of the heat affected zone resulting from large heat inputwelding, which can realize excellent HAZ toughness even in a case ofwelding with a heat input of 20 to 100 kJ/mm for steel plate having athickness of 50 to 80 mm, and a tensile strength of 490 to 570 MPa.

The inventors discovered that by defining the amount of addition of Niand Ni/Mn ratio, the above problems could be advantageously solved. Theyengaged in extensive study and thereby completed the present inventionfor the first time. The gist thereof is as follows:

(1) A high-strength thick steel plate excellent in low temperaturetoughness at a heat affected zone resulting from large heat inputwelding characterized by containing, by wt %, 0.03-0.14% of C, 0.30% orless of Si, 0.8-2.0% of Mn, 0.02% or less of P, 0.005% or less of S,0.001-0.040% of Al, 0.0010-0.0100% of N, 0.8-4.0% of Ni, 0.005-0.030% ofTi, and 0.003-0.040% of Nb, where Ni and Mn satisfy Equation [1], and abalance of iron and unavoidable impurities:Ni/Mn≧10×Ceq−3 (0.36<Ceq<0.42)  [1]

where, Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15

(2) A high-strength thick steel plate excellent in low temperaturetoughness at a heat affected zone resulting from large heat inputwelding as set forth in (1), characterized by further containing, by wt%, one or more of 0.0003-0.0050% of Ca, 0.0003-0.0050% of Mg,0.001-0.030% of an REM and containing at least 100/mm² of grains of anoxide containing 0.0010-0.0050% of O and having an equivalent circlediameter of 0.005 to 0.5 μm.

(3) A high-strength thick steel plate excellent in low temperaturetoughness at a heat affected zone resulting from large heat inputwelding as set forth in (1) or (2), characterized by further containing,by wt %, 0.0005-0.0050% of B.

(4) A high-strength thick steel plate excellent in low temperaturetoughness at a heat affected zone resulting from large heat inputwelding as set forth in any one of (1) to (3), characterized by furthercontaining, by wt %, one or more of 0.1-0.5% of Cr, 0.01-0.5% of Mo,0.005-0.10% of V, and 0.1-1.0% of Cu.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of a welding heat cycle corresponding to 45 kJ/mm.

FIG. 2 is a graph of the relationships among Ni/Mn, Ceq, and thesimulated HAZ toughness.

FIG. 3 is a graph of an effect of improvement of the simulated HAZtoughness due to dispersion of fine oxides or B addition.

BEST MODE FOR CARRYING OUT THE INVENTION

A detailed explanation of the present invention will be given below.

Hitherto, as the means for improving the HAZ toughness, as previouslymentioned, it has been considered to suppress an austenite grain growthat a high temperature. The most effective method as that means ispinning an austenite grain boundaries by dispersed particles so as tostop the movement of the grain boundaries. This pinning is extremelyeffective for reduction of the reheated austenite grain size at the HAZeven in the case where a large heat input such as 20 to 100 kJ/mm isapplied. However, in steel material wherein the amount of alloy elementsadded is increased in order to raise the strength and a carbonequivalent (Ceq) indicating both the weldability of the steel and aquench bardenability in terms of chemical composition becomes 0.36 ormore, the hardness of the HAZ becomes higher. Therefore, there arisesthe new problem that a sufficient HAZ toughness cannot be obtained evenwhen the re-heated austenite grains become finer due to the pinning. Inthe case where the hardness of the HAZ becomes high, it is necessary toimprove the toughness of the base material per se.

Therefore, the inventors intensively studied the optimal chemicalcomposition for improving the toughness of the base material itself forimprovement of the HAZ toughness in the case where the Ceq was as highas 0.36 to 0.42, which becomes necessary for the high-strength thicksteel plate. It has been conventionally known that Ni is an effectiveelement improving the toughness of the matrix. However, it has not beenknown whether it is effective for improving the HAZ toughness in case ofa high Ceq of 0.36 to 0.42. Therefore, the inventors first studied theinfluence of the amount of the Ni addition. For the study, theypredetermined that an 0.003% or more of Nb addition is effective forsecuring the base material strength. For evaluation of the HAZtoughness, they employed the ductility/brittleness transitiontemperature in a Charpy impact test (vTrs) when imparting a heat cyclecorresponding to electro-gas welding (heat input of 45 kJ/mm) shown inFIG. 1.

As a result of their studying the influence of the amount of addition ofNi, first they learned that the required toughness could not be obtainedwhen the Ni was less than 0.8%. Further, even when the Ni was 0.8% ormore, they observed cases where the HAZ toughness was not improved andcases where conversely the HAZ toughness was lowered. Therefore, theyengaged in further intensive studies including the relationships withthe other added elements and Ceq. As a result, they discovered that theHAZ toughness was related to the Ceq and Ni/Mn ratio as shown in FIG. 2when the Ceq is 0.36 to 0.42 in this way. FIG. 2 plots the synthetic HAZtoughness (vTrs) of the steel material used for the study classified foreach Ceq with the Ni/Mn ratio plotted on the abscissa. From FIG. 2, insteel wherein the following relationship stands:Ni/Mn≧10×Ceq−3  [1]a good toughness in terms of vTrs of −15° C. or less was obtained. Asthe reason why steel not satisfying equation [1] cannot obtain asufficient HAZ toughness, it can be considered that the amount ofaddition of Ni is not sufficient and the effect of increasing thetoughness of the matrix is small, or that even if a large amount of Niis contained, MA is produced in the HAZ due to an excessive addition ofMn and the effect of Ni of increasing the toughness disappears. Notethat the inventors engaged in a similar study on the heat cycle of theheat input corresponding to a heat input of 100 kJ/mm for the steel usedin the above described study, and as a result confirmed that a goodsynthetic HAZ toughness is obtained in steel satisfying the relationshipof equation [1] even in the case of a heat input of 100 kJ/mm.

By the above mentioned studies, the inventors found that the HAZtoughness was improved by the addition of Ni in an amount of 0.8% ormore satisfying equation [1]. The inventors further studied regardingthe improvement of the HAZ toughness. They studied the following threemethods as methods of improving the HAZ toughness. First is the methodof suppressing the coarsening of the austenite grains at a hightemperature, since in large heat input welding, the holding time at ahigh temperature becomes a long period. Therefore, the austenite grainscoarsen, which lowers the HAZ toughness. Second is the method ofsuppressing the coarsening of the grain boundary ferrite, since thecooling time after the welding is long in large heat input welding, sothe ferrite generated from the austenite grain boundaries coarsen. Thecoarse grain boundary ferrite becomes the cause of a drop in the HAZtoughness. Third is the method of refining the HAZ microstructureitself.

Concerning the first method of suppressing the coarsening of theaustenite grains, for example, as described in Japanese UnexaminedPatent Publication (Kokai) No. 10-183295, the method of dispersing fineoxides is effective. In this publication, in the dispersion of the fineoxides, the amount of dissolved oxygen in the steel melt is adjusted byan equilibrium reaction with Si in the deoxidation process. Further, thefollowing deoxidation is conducted in a sequence of Ti, Al, and Ca.Then, by this method, oxides having a particle size of 0.01-1.0 μm aredispersed to 5×10³ to 1×10⁵/mm².

Therefore, the inventors intensively studied a method of furtherimproving the HAZ toughness by dispersing fine oxides in a processcontaining 0.03% of Nb and adding 0.8% or more of Ni in the case of aCeq as high as 0.36 to 0.42. First, for the method of dispersing fineoxides, they found the fact that, in such a process, by adjusting theamount of dissolved oxygen of the steel melt to 0.0010-0.0050% in thedeoxidation process, then first deoxidizing the steel melt with Ti, andthen deoxidizing the steel melt with Al, and further adding one or moreof Ca, Mg, and REM, it is possible to disperse fine oxide particleshaving an equivalent circle diameter of 0.005 to 0.5 μm to 100/mm² ormore. Further, by this dispersion of the fine oxides, the coarsening ofthe austenite grains at the holding time at a high temperature in thewelding was suppressed, and the HAZ toughness could be further improved.As an example, the result of comparison with the HAZ toughness obtainedby only adding a proper amount of Ni is shown in FIG. 3. Note that, thelarger the amount of Ni, the finer the produced oxides, and the largerthe number of particles. When the amount of Ni is 1.5% or more, it evenbecomes 1000/mm² or more. This is discovered this time. Further, for theamount of Si in the steel melt, the larger the amount of Si, the harderthe oxide to form. Therefore, it was clarified from this study that theamount of Si was preferably 0.30% or less and further preferably 0.20%or less. On the other hand, in the case where the amount of dissolvedoxygen before the Ti deoxidation exceeds 0.050% and the case where thesequence of the deoxidation elements is different, the oxide coarsensand the sufficient amount of fine oxide cannot be obtained. Therefore,almost no effect of suppression of coarsening of the austenite grainscan be obtained. Note that the number of grains of the oxides having theequivalent circle diameter of 0.005 to 0.5 μm was obtained by preparingan extraction replica from the steel plate as the base material.Observing this under an electron microscope with ×10000 magnification inat least 100 fields (10000 μm² or larger observation area), andobserving particles less than 0.1 μm by properly raising themagnification. The inventors conducted elemental analysis at eachparticle having a diameter of 0.005 to 0.5 μm observed and counted theoxide particles.

Next, the inventors intensively studied the suppression of thecoarsening of the grain boundary ferrite and the refining of the HAZmicrostructure as above described second and third methods ofimprovement of the HAZ toughness. As a result, the inventors clarifiedthat the addition of B was effective particularly in the case wherelarge heat input welding corresponding to 20 to 100 kJ/mm was conductedin a process where the Ceq was as high as 0.36 to 0.42 and Ni was addedin an amount of 0.8% or more. The reason for that, in terms of thesuppression of the coarsening of the grain boundary ferrite, is thesuppression of the production of grain boundary ferrite by a segregationof the solute B at the re-heated austenite grain boundaries. Further, interms of the increased refined HAZ microstructure, in the case where thecooling rate is slow in the large heat input welding, the B nitridesprecipitated at the austenite grain boundaries and in the inclusions inthe austenite grains due to the addition of B, and a large number offine ferrite grains of several micrometers using those as nuclei arepresent at the austenite grain boundaries and in the grains, whereby theHAZ structure is made finer. The inventors compared the improvement ofthe HAZ toughness by the addition of B with the HAZ toughness obtainedby only properly adding Ni. The results are shown in FIG. 3. It is seenthat the HAZ toughness is further improved by the addition of B.Further, FIG. 3 shows the HAZ toughness in the case where the B is addedin addition to the method of dispersing the above mentioned fine oxides.The HAZ toughness is further improved by the dispersion of the fineoxides and the addition of B. It is considered that the improvement isdue to the increase of the oxides acting as precipitation sites of theBN and thereby the greater refined HAZ microstructure due to theincrease of the ferrite using the BN as the nuclei.

Further, from the viewpoints of securing the strength and improvement ofthe corrosion resistance, in addition to the above described conditions,the inventors also studied the HAZ toughness when Cu, Cr, Mo, and V wereadded. As a result, they found that the HAZ toughness was not greatlylowered when they were added within ranges of 0.1-0.4%, 0.1-0.5%,0.03-0.2%, and 0.005-0.050%.

Note that, the method of production of the steel plate according to thepresent invention is not particularly limited. The steel plate may beproduced by any known method. For example, a slab is formed from steelmelt adjusted to the preferred composition described above by acontinuous casting method, then is heated to 1000 to 1250° C., then ishot rolled.

Next, an explanation will be given on the reasons for limitation of theingredients of the steel materials used in the present invention. Below,the wt % in the compositions will be simply described as %.

C is an ingredient effective for improving the strength of the steel, sothe lower limit is made 0.03%. An excess addition produces large amountsof carbide and MA and remarkably lowers the HAZ toughness, therefore theupper limit was made 0.14%.

Si is an ingredient necessary for securing the strength of the basematerial and deoxidation, but in order to prevent the drop in thetoughness due to the hardening of the HAZ, the upper limit was made0.30%. When utilizing an oxide, the upper limit of the content is made0.20% or less in order to prevent the reduction of the oxygenconcentration in the molten steel.

Mn is an ingredient effective for securing the strength and toughness ofthe base material and must be added in an amount of 0.8% or more, butthe upper limit was made 2.0% within the range where the toughness,cracking property etc. of the welding zone were permissible. Further,concerning the upper limit of Mn, it is necessary to satisfy equation[1] indicating the relationship among the Ceq, Mn amount, and the Niamount. This is based on the newly found fact by this study that theincrease of Mn becomes the cause of production of a large amount of MAin the HAZ microstructure in the case where the Ceq is high and theeffect of improvement of the HAZ toughness by Ni disappears.Ni/Mn≧10×Ceq−3  [1]

P is desirably as little as possible, but in order to reduce thisindustrially, enormous costs are entailed, so the range of content wasmade 0.02 or less.

S is desirably as little as possible, but in order to reduce thisindustrially, enormous costs are entailed, so the range of content wasmade 0.005 or less.

Ni is an important element in the present invention and must be added inan amount of at least 0.8%. Further, concerning the lower limit of Ni,it is necessary to satisfy equation [1] showing the relationship of Ceq,the amount of Mn, and the amount of Ni. The upper limit was made 4.0%from the viewpoint of the production cost.Ni/Mn≧10×Ceq−3  [1]

Nb is an element effective for improving the strength of the basematerial by improving the quench bardenability, so is added in an amountof 0.003% or more. However, if a lot of Nb is added, the MA becomes easyto be produced in the HAZ regardless of the Ni/Mn ratio, while when itis added in an amount larger than 0.040%, a large amount of coarse MAhaving a long axis of 5 μm is produced in the HAZ and greatly reducesthe HAZ toughness. Therefore, the upper limit of Nb is made 0.040%. Notethat, in order to obtain a higher toughness, preferably the amount of Nbis suppressed to 0.020% or less where almost no coarse MA having a longaxis of 5 μm is produced in the case of the Ni/Mn ratio satisfying theabove mentioned equation [1]. In order to stably obtain further highertoughness, it is preferred to suppress the amount of Nb to 0.010% orless where almost no MA having a long axis of 3 μm or more is generatedin the case of an Ni/Mn ratio satisfying the above mentioned equation[1].

Al is an important deoxidation element, so the lower limit was made0.001%. Further, when a large amount of Al is present, the surfacequality of the slab is deteriorated, so the upper limit was made 0.040%.

Ti is added in an amount of 0.005% or more according to need in order toproduce the Ti nitride and the Ti-containing oxide particles whichbecome pinning sites necessary for suppressing the coarsening of there-heated austenite grains. However, its excess addition increases theamount of dissolved Ti and induces a drop in the HAZ toughness,therefore 0.030% was made the upper limit.

N is adjusted in the amount of addition, if necessary, in order toproduce the Ti nitride and the B nitride particles at the austenitegrain boundaries and in the grains during the cooling after the welding.In order to form the B nitride by binding with B, N must be added in anamount of 0.0010% or more, but its excess addition increases the amountof dissolved N and induces a drop in the HAZ toughness, so 0.0100% wasmade the upper limit.

Ca is added in an amount of 0.0003% or more, if necessary, in order toproduce the Ca-based oxide particles acting as pinning grains necessaryfor suppressing the coarsening of the re-heated austenite grains.However, excess addition produces coarse inclusions, so 0.0050% was madethe upper limit.

Mg is added in an amount of 0.0003% or more, if necessary, in order togenerate the Mg-based oxide particles acting as pinning grains necessaryfor suppressing the coarsening of the re-heated austenite grains.However, excess addition produces coarse inclusions, so 0.0050% was madethe upper limit.

A REM is added in the amount of 0.0001% or more, if necessary, in orderto produce the REM-based oxide particles acting as pinning sitesnecessary for suppressing the coarsening of the re-heated austenitegrains. However, excess addition produces coarse inclusions, so 0.030%was made the upper limit. Further, the “REM” mentioned here represent Ceand La, and the amount of addition is the total amount of the two.

B is added in an amount of 0.0005% or more, if necessary, in order tocause the dissolved B to segregate at the austenite grain boundariesduring the cooling after the welding and suppress the production of thegrain boundary ferrite and further to produce BN at the austenite grainboundaries and in the grains. However, its excess addition increases theamount of dissolved B, greatly raises the HAZ hardness, and induces adrop in the HAZ toughness, so 0.0050% was made the upper limit.

Cu is added in an amount of 0.1% or more, if necessary, in order toimprove the strength and corrosion resistance of the steel. The effectthereof is saturated at 1.0%, so the upper limit was made 1.0, but whenit exceeds 0.4%, MA becomes easy to be generated and the HAZ toughnessis lowered, therefore 0.4% or less is preferred.

Cr is added in an amount of 0.1% or more, if necessary, in order toimprove the corrosion resistance of the steel, but its excess additioninduces a drop in the HAZ toughness due to the generation of MA, so 0.5%was made the upper limit.

Mo is an element effective for improving the strength and the corrosionresistance of the base material and is added in an amount of 0.01% ormore, if necessary. The effect thereof is saturated at 0.5%, so theupper limit was made 1.0, but its excess addition induces a drop in theHAZ toughness due to the generation of MA, so 0.2% or less is preferred.

V is an element effective for improving the strength of the basematerial and is added in an amount of 0.005% or more, if necessary. Theeffect thereof is saturated at 0.5%, so the upper limit was made 0.10%,but its excess addition induces a drop in the HAZ toughness due to thegeneration of MA, so 0.050% or less is preferred.

EXAMPLE 1

Slabs were prepared by continuously casting the steel melt having thechemical compositions shown in Table 1. For D23-D31 and D46-D49, theamounts of dissolved oxygen of the steel melt were adjusted to0.0010%-0.0050% by Si before charging the Ti, then Ti was used fordeoxidization, then Al was used for deoxidation, then any of Ca, Mg, orREM was added for deoxidation. The slabs were re-heated at 1100 to 1250°C., then were hot rolled by the following two methods to produce steelplates having plate thicknesses of 50 to 80 mm. One method was to rollthe plate at a surface temperature within a range of 750-900° C., thencool it by water at a plate surface temperature within the temperaturerange of 200-400° C. after recalescence (described as TMCP in Table 2).The other method of production is cooling with water down to roomtemperature after hot rolling, then tempering within a range of 500-600°C. (described as DQ-T in Table 2).

Table 2 shows the production conditions, plate thicknesses, andmechanical properties of the steel plates. Further, for D23-D31 andD46-D49, the numbers of fine oxide particles having equivalent circlediameters of 0.005 to 0.5 μm measured at any location of the steelplates were additionally described. The number of the oxide particlesare found by preparing an extraction replica from any portion of thesteel plate, observing this under an electron microscope with X10000magnification in 100 fields or more (10000 μm² or more in observationarea), and observing particles less than 0.1 μm by properly raising themagnification. Elemental analysis was conducted for each observedparticle having a diameter of 0.005 to 0.5 μm and the oxide particleswere counted. All of the steel plates among D23-D31 and D46-D49 had fineoxide particles having equivalent circle diameters of 0.01 to 0.5 μmdispersed to 100/mm² within the range of the present invention. Notethat it is seen from the comparison of D46 and D47 and D48 and D49wherein elements other than Si are almost equal, that the smaller theamount of Si, i.e. 0.20% or less, the larger the amount of the oxides.

Each of these steel plates was made to abut against another steel plateand subjected to vertical one-pass butt welding in using electro-gaswelding (EGW) or electro-slag welding (ESW) having welding heat inputsof 20 to 100 kJ/mm. Then, in the HAZ located at the center portion ofthe plate thickness (t/2), notches were formed at two locations, thatis, the HAZ separated from the FL (Fusion Line) by 1 mm (HAZ 1 mm) andthe FL. A Charpy impact test was conducted at −40° C. Table 2 shows thewelding conditions and HAZ toughnesses. In the Charpy impact test here,use was made of JIS No. 4 2 mm V-notch full size test pieces. Further,Table 2 also describes the former austenite grain size in FL-HAZ 1 mm.The “former austenite grain size in FL-HAZ 1 mm” described here is theaverage grain size obtained by measuring the grain size of the formeraustenite grains contained within a 2 mm range in the thicknessdirection centered by t/2 and the FL-HAZ 1 mm range by thecross-sectional method. Note that, the measurement was conducted byusing particulate ferrite connected in a form of a net as the grainboundaries of the former austenite grains.

D1 to D49 are steels of the present invention. The chemical compositionsof the steels are properly controlled, therefore the large heat inputHAZ toughness at −40° C. is good while satisfying the predetermined basematerial performances. Further, in D23-D31 and D46-D49 obtained bydispersing the fine oxide particles, the former austenite grain size inFL-HAZ 1 mm becomes finer than those of the others, i.e. 200 μm or less,and the large heat input HAZ toughness at −40° C. becomes furtherhigher. Further, D20 aiming at increasing the refined HAZ structure byadding B has a better HAZ toughness in comparison with D19 withoutaddition of B and containing addition elements other than B in the equalamounts and exhibits a higher performance for also the large heat inputHAZ toughness at −40° C.

On the other hand, the Comparative Steels C1 to C17 do not containsufficient Ni for satisfying equation [1] or properly control thechemical compositions of the steels, so the large heat input HAZtoughness is insufficient. TABLE 1 Class Sym. C Si Mn P S Ni Nb Al Ti NCa Mg Inv. D1 0.04 0.13 1.31 0.008 0.002 1.6 0.006 0.015 0.008 0.0035steel D2 0.04 0.17 0.81 0.008 0.002 2.8 0.006 0.015 0.008 0.0035 D3 0.070.17 1.40 0.007 0.002 0.9 0.006 0.015 0.007 0.0035 D4 0.07 0.05 0.810.006 0.002 2.4 0.006 0.014 0.008 0.0035 D5 0.10 0.19 1.11 0.007 0.0021.1 0.004 0.015 0.007 0.0032 D6 0.13 0.05 0.91 0.007 0.003 1.2 0.0050.014 0.006 0.0037 D7 0.06 0.19 1.14 0.007 0.002 1.3 0.005 0.012 0.0060.0042 D8 0.06 0.22 1.11 0.006 0.003 2.1 0.005 0.014 0.006 0.0035 D90.09 0.15 1.21 0.007 0.003 1.4 0.006 0.015 0.006 0.0035 D10 0.12 0.111.01 0.007 0.002 1.4 0.005 0.014 0.009 0.0035 D11 0.13 0.14 0.91 0.0060.002 1.5 0.006 0.015 0.008 0.0035 D12 0.06 0.14 1.41 0.007 0.002 1.60.005 0.014 0.008 0.0032 D13 0.06 0.22 1.11 0.007 0.002 2.4 0.005 0.0140.007 0.0035 D14 0.09 0.21 1.31 0.008 0.002 1.4 0.005 0.014 0.008 0.0035D15 0.13 0.19 1.01 0.007 0.002 1.6 0.005 0.014 0.007 0.0035 D16 0.120.22 0.91 0.007 0.003 2.1 0.006 0.015 0.006 0.0035 D17 0.13 0.17 1.010.006 0.002 1.7 0.005 0.014 0.006 0.0032 D18 0.12 0.15 1.11 0.008 0.0021.5 0.005 0.014 0.006 0.0037 D19 0.06 0.14 1.41 0.007 0.002 1.6 0.0050.014 0.009 0.0041 D20 0.06 0.14 1.40 0.007 0.002 1.6 0.005 0.014 0.0090.0041 D21 0.06 0.10 1.21 0.007 0.002 1.7 0.006 0.015 0.010 0.0035 D220.06 0.22 1.11 0.007 0.002 1.8 0.005 0.014 0.008 0.0055 D23 0.06 0.161.31 0.008 0.004 1.8 0.005 0.014 0.008 0.0035 0.0018 D24 0.06 0.17 1.400.008 0.004 1.6 0.005 0.014 0.008 0.0037 0.0019 D25 0.06 0.12 1.20 0.0070.002 2.1 0.006 0.015 0.007 0.0033 0.0016 D26 0.06 0.15 1.00 0.007 0.0022.6 0.005 0.014 0.008 0.0036 D27 0.06 0.11 1.21 0.007 0.002 1.8 0.0150.015 0.007 0.0035 0.0014 D28 0.06 0.23 1.00 0.007 0.002 1.9 0.005 0.0140.006 0.0035 0.0018 D29 0.06 0.17 1.20 0.008 0.004 1.8 0.005 0.014 0.0060.0035 0.0036 D30 0.06 0.12 1.20 0.007 0.002 1.7 0.006 0.015 0.0060.0035 0.0009 D31 0.06 0.12 1.20 0.007 0.002 1.9 0.006 0.015 0.0060.0035 0.0016 D32 0.07 0.15 1.21 0.007 0.003 1.4 0.028 0.014 0.0090.0032 0.0015 D33 0.08 0.13 1.21 0.009 0.002 1.3 0.013 0.014 0.0100.0052 0.0017 D34 0.08 0.19 1.21 0.008 0.003 1.5 0.013 0.025 0.0080.0076 0.0012 D35 0.08 0.19 1.21 0.010 0.002 1.2 0.021 0.014 0.0080.0035 Class REM O B Cu Cr Mo V Ceq Nl/Mn 10 × Ceq-3 Judg.* Inv. 0.371.22 0.7 ◯ steel 0.36 3.46 0.6 ◯ 0.36 0.64 0.6 ◯ 0.37 2.96 0.7 ◯ 0.360.99 0.6 ◯ 0.36 1.32 0.6 ◯ 0.38 0.92 0.8 ◯ 0.39 1.89 0.9 ◯ 0.39 1.16 0.9◯ 0.38 1.39 0.8 ◯ 0.38 1.65 0.8 ◯ 0.40 1.13 1.0 ◯ 0.41 2.16 1.1 ◯ 0.401.07 1.0 ◯ 0.41 1.58 1.1 ◯ 0.41 2.31 1.1 ◯ 0.41 1.68 1.1 ◯ 0.41 1.35 1.1◯ 0.40 1.13 1.0 ◯ 0.0012 0.40 1.14 1.0 ◯ 0.4 0.40 1.40 1.0 ◯ 0.0023 0.20.41 1.62 1.1 ◯ 0.0019 0.40 1.37 1.0 ◯ 0.0019 0.0008 0.40 1.14 1.0 ◯0.0017 0.0009 0.40 1.75 1.0 ◯ 0.0220 0.0020 0.0011 0.40 2.60 1.0 ◯0.0017 0.3 0.40 1.49 1.0 ◯ 0.0030 0.0009 0.2 0.017 0.40 1.90 1.0 ◯0.0029 0.0009 0.3 0.40 1.50 1.0 ◯ 0.0028 0.0009 0.2 0.05 0.40 1.42 1.0 ◯0.0023 0.0009 0.2 0.037 0.41 1.58 1.1 ◯ 0.0018 0.0012 0.4 0.39 1.16 0.9◯ 0.0020 0.0009 0.3 0.39 1.07 0.9 ◯ 0.0015 0.0035 0.4 0.41 1.24 1.1 ◯0.36 0.99 0.6 ◯ Class Sym. C Si Mn P S Ni Nb Al Ti N Ca Mg Inv. D36 0.090.19 1.21 0.010 0.003 1.5 0.015 0.015 0.008 0.0035 steel D37 0.10 0.191.11 0.006 0.002 1.9 0.015 0.035 0.007 0.0035 D38 0.11 0.14 0.91 0.0060.003 1.7 0.031 0.014 0.008 0.0035 D39 0.12 0.14 1.11 0.008 0.002 1.80.029 0.032 0.007 0.0032 D40 0.10 0.14 1.21 0.009 0.002 1.6 0.030 0.0140.006 0.0037 D41 0.10 0.24 1.21 0.006 0.002 1.5 0.031 0.028 0.006 0.0042D42 0.08 0.24 1.31 0.005 0.003 1.6 0.032 0.014 0.006 0.0035 D43 0.030.22 1.31 0.008 0.003 2.5 0.035 0.038 0.006 0.0035 D44 0.06 0.24 0.810.007 0.003 3.2 0.035 0.014 0.009 0.0035 D45 0.03 0.11 1.51 0.007 0.0021.8 0.035 0.014 0.010 0.0035 D46 0.07 0.11 1.21 0.007 0.003 1.4 0.0280.014 0.009 0.0032 0.0015 D47 0.07 0.28 1.21 0.007 0.003 1.4 0.028 0.0140.009 0.0032 0.0015 D48 0.06 0.11 1.20 0.008 0.004 1.8 0.005 0.014 0.0060.0035 0.0018 D49 0.06 0.28 1.20 0.008 0.004 1.8 0.005 0.014 0.0060.0035 0.0017 Comp. C1 0.04 0.14 1.90 0.007 0.002 0.1 0.008 0.012 0.0080.0032 Steel C2 0.04 0.09 1.60 0.006 0.003 0.8 0.009 0.019 0.008 0.0037C3 0.06 0.09 1.70 0.007 0.003 0.2 0.005 0.012 0.008 0.0042 C4 0.09 0.111.60 0.008 0.003 0.0 0.006 0.015 0.007 0.0035 C5 0.10 0.14 1.30 0.0080.002 0.7 0.005 0.014 0.008 0.0035 C6 0.13 0.22 1.20 0.007 0.002 0.50.003 0.012 0.008 0.0035 C7 0.06 0.11 1.90 0.007 0.002 0.1 0.006 0.0150.008 0.0035 C8 0.06 0.14 1.60 0.007 0.004 0.8 0.005 0.014 0.007 0.0032C9 0.09 0.17 1.40 0.008 0.002 0.9 0.005 0.014 0.008 0.0037 C10 0.11 0.231.30 0.007 0.002 0.8 0.006 0.015 0.007 0.0042 C11 0.06 0.16 2.00 0.0080.004 0.1 0.005 0.014 0.006 0.0035 C12 0.06 0.11 1.60 0.007 0.002 1.10.006 0.015 0.006 0.0035 C13 0.10 0.25 1.70 0.006 0.003 0.2 0.006 0.0150.006 0.0035 C14 0.12 0.14 1.40 0.007 0.002 0.7 0.005 0.014 0.006 0.0035C15 0.09 0.12 1.60 0.008 0.003 0.8 0.015 0.013 0.009 0.0032 C16 0.080.24 1.50 0.008 0.003 0.6 0.035 0.013 0.010 0.0037 C17 0.09 0.14 1.200.007 0.002 1.4 0.045 0.014 0.008 0.0042 Class REM O B Cu Cr Mo V CeqNi/Mn 10 × Ceq-3 Judg.* Inv. 0.03 0.40 1.24 1.0 ◯ steel 0.41 1.71 1.1 ◯0.045 0.38 1.87 0.8 ◯ 0.43 1.62 1.3 ◯ 0.2 0.42 1.32 1.2 ◯ 0.40 1.24 1.0◯ 0.41 1.22 1.1 ◯ 0.42 1.91 1.2 ◯ 0.41 3.95 1.1 ◯ 0.40 1.19 1.0 ◯ 0.00180.4 0.39 1.16 0.9 ◯ 0.0018 0.4 0.39 1.16 0.9 ◯ 0.0026 0.0012 0.3 0.401.50 1.0 ◯ 0.0026 0.0012 0.3 0.40 1.50 1.0 ◯ Comp. 0.36 0.05 0.6 X steel0.36 0.50 0.6 X 0.36 0.12 0.6 X 0.36 0.00 0.6 X 0.36 0.54 0.6 X 0.360.42 0.6 X 0.38 0.05 0.8 X 0.38 0.50 0.8 X 0.38 0.64 0.8 X 0.38 0.62 0.8X 0.40 0.05 1.0 X 0.40 0.69 1.0 X 0.40 0.12 1.0 X 0.40 0.50 1.0 X 0.410.50 1.1 X 0.37 0.40 0.7 X 0.38 1.17 0.8 ◯*◯ is described where Ni/Mn ≧ 10 × Ceq-3 is satisfied, and X isdescribed where it is not satisfied.

TABLE 2 Plate Base material (t/2)¹⁾ Number²⁾ Produc- thick- TensileYield of oxide tion ness strength stress particles Class Symbol method(mm) (Mpa) (Mpa) vE_(−40(J)) (per mm²) Inv. D1 TMCP 60 576 476 231 steelD2 TMCP 65 565 465 229 D3 DQ-T 70 576 456 225 D4 TMCP 60 576 476 231 D5DQ-T 55 605 485 238 D6 TMCP 65 565 465 229 D7 TMCP 70 560 460 219 D8TMCP 80 541 441 213 D9 DQ-T 60 601 481 225 D10 TMCP 65 570 470 223 D11TMCP 75 550 450 216 D12 TMCP 80 545 445 208 D13 TMCP 55 596 496 224 D14DQ-T 65 595 475 217 D15 TMCP 70 566 466 213 D16 TMCP 65 578 478 214 D17DQ-T 70 588 468 211 D18 TMCP 75 556 456 210 D19 TMCP 70 565 465 214 D20TMCP 70 575 482 214 D21 DQ-T 70 585 465 214 D22 TMCP 70 566 466 213 D23TMCP 65 575 475 218 900 D24 DQ-T 60 605 485 221 1200 D25 TMCP 70 565 465214 1300 D26 TMCP 80 545 445 209 1100 D27 TMCP 70 565 465 214 900 D28TMCP 65 574 474 218 1800 D29 TMCP 60 585 485 221 2100 D30 DQ-T 65 594474 218 2400 D31 TMCP 60 587 487 219 1900 D32 TMCP 70 563 463 217 700D33 TMCP 65 572 472 221 600 D34 DQ-T 80 567 447 207 1400 D35 TMCP 70 555455 225 one-pass butt welding condition³⁾ Average γ grain HAZtoughness⁵⁾ Welding Heat input size in FL-HAZ 1 FL/vE⁻⁴⁰ FL + 1 mm/Class method (kJ/mm) mm (μm)⁴⁾ (J) vE⁻⁴⁰(J) Inv. EGW 39 480 140 128steel EGW 42 520 135 124 ESW 85 770 116 106 ESW 73 660 123 113 ESW 67605 127 117 EGW 42 520 135 124 ESW 85 770 116 106 EGW 51 640 124 114 EGW39 480 140 128 ESW 79 715 119 109 EGW 48 600 128 117 EGW 51 640 124 114EGW 35 440 144 132 ESW 79 715 119 109 ESW 85 770 116 106 EGW 42 520 135124 EGW 45 560 131 120 EGW 48 600 128 117 EGW 45 560 131 120 EGW 45 560171 156 ESW 85 770 116 106 EGW 45 560 184 180 ESW 79 180 207 189 EGW 39165 214 196 EGW 45 152 221 203 EGW 51 185 204 187 ESW 45 180 207 189 ESW79 167 213 195 EGW 39 184 205 188 EGW 42 165 214 196 EGW 39 184 205 188EGW 45 180 207 189 EGW 42 164 214 197 ESW 98 180 196 180 ESW 85 660 123113 Plate Base material (t/2)¹⁾ Number²⁾ Produc- thick- Tensile Yield ofoxide tion ness strength stress particles Class Symbol method (mm) (Mpa)(Mpa) vE_(−40(J)) (per mm²) Inv. D36 DQ-T 70 584 464 215 steel D37 TMCP65 578 478 214 D38 TMCP 60 581 481 226 D39 TMCP 70 571 471 208 D40 DQ-T80 570 450 203 D41 TMCP 70 565 465 214 D42 TMCP 65 576 476 216 D43 TMCP60 589 489 217 D44 TMCP 65 577 477 215 D45 DQ-T 60 605 485 221 D46 TMCP70 553 465 217 900 D47 TMCP 70 579 481 217 400 D48 TMCP 60 578 485 2212300 D49 TMCP 60 592 485 221 1500 Comp. C1 TMCP 70 556 456 225 steel. C2DQ-T 60 595 475 233 C3 TMCP 75 544 444 224 C4 TMCP 60 574 474 234 C5TMCP 60 576 476 232 C6 TMCP 55 586 486 236 C7 DQ-T 60 601 481 226 C8TMCP 60 580 480 227 C9 TMCP 60 581 481 226 C10 TMCP 60 580 480 227 C11DQ-T 70 585 465 214 C12 TMCP 60 585 485 221 C13 TMCP 70 564 464 215 C14TMCP 65 575 475 218 C15 TMCP 55 598 498 222 C16 DQ-T 65 588 468 226 C17TMCP 60 581 481 226 one-pass butt welding condition³⁾ Average γ grainHAZ toughness⁵⁾ Welding Heat input size in FL-HAZ 1 FL/vE⁻⁴⁰ FL + 1 mm/Class method (kJ/mm) mm (μm)⁴⁾ (J) vE⁻⁴⁰(J) Inv. ESW 85 605 127 117steel EGW 42 520 135 124 ESW 73 770 116 106 EGW 45 640 124 114 EGW 51480 140 128 ESW 85 715 119 109 EGW 42 600 128 117 EGW 39 640 124 114 EGW42 440 144 132 ESW 73 715 119 109 EGW 45 145 225 206 EGW 45 195 200 183EGW 39 164 214 236 EGW 39 185 204 225 Comp. ESW 85 770 36 25 steel EGW39 480 57 40 EGW 48 600 46 32 EGW 39 480 57 40 EGW 39 480 57 40 ESW 67605 45 32 EGW 39 480 57 40 EGW 39 480 57 40 EGW 39 480 57 40 EGW 39 48057 40 ESW 85 770 36 25 EGW 39 480 57 40 EGW 45 560 49 34 EGW 42 520 5337 ESW 67 605 45 32 EGW 42 502 53 37 EGW 39 480 57 40¹⁾Plate thickness center position; YS and TS are average values of twotest pieces; Charpy absorption energy at −40° C. (vE−40) is an averagevalue of three test pieces.²⁾Extraction replica was prepared from any portion of steel plate, andobserved under electron microscope by ×10000 magnification in 100 fieldsor more (10000 μm² or more in observation area). However, particles ofless than 0.1 μm were obtained at properly raising magnification.# Particles are counted including oxide by elemental analysis amongparticles of equivalent circle diameter of 0.005 to 0.5 μm and convertto number per 1 mm².³⁾EGW: Electro-gas welding; ESW: Electro-slag welding; Welding heatinput is an average value in total length of welding; Common weldingmaterial is used in welding processes⁴⁾Average grain size of former austenite grains contained within a 2 mmrange in the thickness direction centered by t/2 and the FL-HAZ 1 mmrange was measured by the cross-sectional method. Measurement wasconducted by using particulate ferrite connected in a form of a net asthe grain boundaries of the former austenite grains.⁵⁾FL notches are layed down so as to equally divide WM and HAZ. vE − 40-at each notch location is an average value of three test pieces.As described above, the present invention provides thick steel platesatisfying the excellent toughness demands regarding destruction forships, offshore structures, medium/high rise buildings, etc.

1. A high-strength thick steel plate excellent in low temperaturetoughness at heat affected zone resulting from large heat input welding,characterized by containing, by wt %, C: 0.03-0.14%, Si: 0.30% or less,Mn: 0.8-2.0%, P: 0.02% or less, S: 0.005% or less, Al: 0.001-0.040%, N:0.0010-0.0100%, Ni: 0.8-4.0%, Ti: 0.005-0.030%, and Nb: 0.003-0.040%,where Ni and Mn satisfy equation [1], and the balance of iron andunavoidable impurities:Ni/Mn≧10×Ceq−3 (0.36<Ceq<0.42)  [1] where,Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15
 2. A high-strength thick steel plateexcellent in low temperature toughness at heat affected zone resultingfrom large heat input welding according to claim 1, characterized byfturther containing, by wt %, one or more of: Ca: 0.0003-0.0050%, Mg:0.0003-0.0050%, and REM: 0.001-0.030% and contains at least 1 00/mm² ofoxide particles containing O: 0.0010-0.0050% and having a equivalentcircle diameter of 0.005 to 0.5 μm.
 3. A high-strength thick steel plateexcellent in low temperature toughness at heat affected zone resultingfrom large heat input welding according to claim 1, characterized byfurther containing, by wt %, B: 0.0005-0.0050%.
 4. A high-strength thicksteel plate excellent in low temperature toughness at heat affected zoneresulting from large heat input welding according to claim 1,characterized by further containing, by wt %, one or more of: Cr:0.1-0.5%, Mo: 0.01-0.5%, V: 0.005-0.10%, and Cu: 0.1-1.0%.